I recently went through the objective caml sources, and printf uses a
module named obj that isn't described in the reference manual.
There were some messages about the Obj module on this list in
January, look at:
http://pauillac.inria.fr/caml/caml-list/0926.html
http://pauillac.inria.fr/caml/caml-list/0929.html
http://pauillac.inria.fr/caml/caml-list/0933.html
http://pauillac.inria.fr/caml/caml-list/0936.html
http://pauillac.inria.fr/caml/caml-list/0975.html
And also
http://pauillac.inria.fr/caml/caml-list/0935.html
might be interesting.
Bye,
Hendrik